Method and system of speech transmission



April 23, 1935. .1 c. STEINBERG METHOD AND SYSTEM OF SPEECH TRANSMISSION Z aj za v v s i3: H kWh-E m N\ g Q 8 oh A m N- INVENTOR J C STE INBERG A TTORNE V Patented Apr. 23, 1935 i I 1,998,824

UNITED STATES OFFICE METHOD AND SYSTEM OF SPEECH TRANSMISSION John QSteinberg, Sparta, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 10, 1932, Serial No. 616,436

4 Claims. (01. 178-44) The present invention relates to the transline, a submarine telephone cable or a line in mission of speech or similar waves, and permits which it is desirable to make eflicient use of the. the use of a narrowed frequency band for transfrequency range by providing additional termission purposes, with the same degree of underxninal apparatus. 4 k v 5 standability and naturalness in the received Line L1 is balanced by the usual network N1 5 speech. and includes hybrid coil 1:11. A balancing net- Articulation tests show that all frequencies work N2 and hybrid coil H2 are provided'ior below 400 or 500 cycles can be eliminated from line L2. 7 speech with practically negligible loss in articu- The transmitting branch T includes a high- 10 lation or understandability compared with the pass filter 2 which permits the transmission of 10 loss caused by elimination of an equal range from all frequency components in excess of some lownear the upper edge of the band usually emer limiting frequency, for example 506 cycles, ployed in commercial telephone transmission but prevents transmission of frequencies below which may be around 2800 cycles or less. It is such limit. This filter in practice may be a usual to transmit frequencies down to 259 cycles, multi-section electrical wave filter of a type well 15 however, on account of the importance of these known in the art. An a p 3 is o Shown low frequencies in imparting naturalness to the in this branch.- The filter i in the line Lz may received speech. The importance of these lower be in all resp identical With filter frequencies seems to be due to the fact that the Receiving branch R is divided into two parallel fundamental or vocal cord frequency in the case pa One Of Which includes merely an p fi 20 of a womans voice and the first harmonic in 8 Where necessary and the'oiher 0f Q h the case of a mans voice lies in this region; cludes a demodulator device Hi and a filter which It is an object of the present invention to peris theconjugafi 0f filters 2 and 13, that a W- mit use of a narrower range in transmission of P ss filtcrv Which Supp s iss o 0f speech while restoring the naturalness of the f qu i in x s i 5 cy l but allows 25 received speech by restoring the frequencies t the transmission of frequencies lower than 500 actually transmitted. cycles. i

A further object of the invention is to raise The device W h has been Ca ed demoduthe efficiency of a, speech transmission li or lator it is any suitable element for deriving from so channel by providing signal channels employing hi her fr q s f speech a q n y some of the frequency range heretofore used for P0118113 Corresponding Speech q c s hat speech, while at the same time maintaining the are eliminated by filter It y fi, O articulation and naturalness oi; the speech transamp e, a Vacuum detector uit or in genrhitted over such line or ch nn l, eral any transmission element which has a non- 135 According to the invention the low frequencies lineal" relation between input and p are eliminated before transmission and are re- It kIlOWfl that Speech 501111015 Which onta stared at a receiving point by deriving th from a fundamental or first harmonic of about 250 the higher frequency components of the speech. y s a Conta -a nu ber of ha monics 01 Th frequency range thu saved Q11 th li can overtones of this same fundamental or basic fre- -l0 be used for telegraph or other forms of low irei 3. Demofllllatol" W r p duceS t s fund& 0 quency signaling or other transmi si n, mental frequency component by deriving the dif- An illustrative example of how the invention Terence frequency between haTmOniCallY e a ed may be practiced is given in the accompanying COmDOIIBHlBS the Speech W v drawing, the single figure of which shows a twc- In ord r to p ovid f r posi signaling n way terminal of a composited telephone line the line L2 2. filter 65 which suppresses the range 45 or channel embodying one form of the invenof frequencies transmitted through filter 4 but tion. allows transmission of lower frequencies serves In the drawing provision is made for repeatto connect line L2 with any suitable low frequency ing, in accordance with the invention, between Signaling Set 56 Such as a low q cy Carrier the subscribers line L1 and the main or toll line telegraph transmitting and receiving Channel 1 50 L2. Speech received over the line Li from the the like. subscribers set i is repeated through the branch In operation speech waves from the subscribers T into line L2. Speech passing in the opposite set i transmitted over line L1 are transmitted in direction is transmitted through the receiving part into branch T. The lower frequencies of branch 3. The line L2 may be a toll telephone the speech Waves are eliminated in high-pass filter 2 so that only those components of the speech Waves in excess of about 500 cycles are allowed to pass through amplifier 3 and into the line L2, by way of hybrid coil H2 and filter 4.

It is found that speech waves transmitted in this manner have lost little, if anything, in articulation or understandability, the main effect of eliminating these low frequency components being loss of naturalness or quality. The manner in which these waves are received is seen by considering the receiving side of the station shown in the drawing, which may be identical with the station at the distant end of line L2.

Waves transmitted in the manner described and received at the station shown pass through filter :l and in part through transformer 5, an plifier 6, and transformer i into the. two paths making up the receiving branch proper. The waves constituting the entire received band pass directly through amplifier 8, transformer l2 and out into the line L1. A portion of the Waves of this same frequency range pass into demodulator it which serves to derive from certain of the harmonically related components lying within this range, as by'producing the beator difference-frequency between such components, a component of fundamental frequency of about 250 cycles in the case of a womans voice or a first harmonic of about this same frequency in the case of a mans voice. If, for example, the received waves at any given instant contain components having frequencies of 1250 cycles and 1500 cycles per second, the demodulator i0 produces from these two frequencies occurring simultaneously a derived component of 250 cycles. This derived component of lower frequency is permitted to pass through low-pass filter ii while the direct transmission of the frequency band received over the line is prevented from passing through this filter. The wave component of such lower frequency is blended with the speech waves from amplifier 8 in the output transformer i2 so that the speech waves finally impressed on line L1 contain the low frequency component corresponding to that which was suppressed at the distant station. The subscriber at station I hears the received speech with a higher degree of naturalness than as though this low frequency component were absent.

It is evident that the frequencies of the fundamental and overtone in the restored portion will be exactly equal to the frequencies that were eliminated from the original speech which vary from instant to instant during utterance. Also the level of the restored portion will vary in accordance with the level of the incoming speech. By properly choosing the characteristics of the low-pass filter and demodulator I U and adjusting the gain of one or both channels in the receiving branch R, the level of the restored portion is blended with the level of the incoming speech. Thus the rhythm and timbre of the original speech may be restored to a degree of perfection depending upon the care in choosing the above characteristics. Just what these characteristics should be in any particular case can best be determined by trial.

Simultaneously with and independently of the speech transmission as above described, other kinds of signaling may take place over line L2 from the terminal apparatus IS in the manner generally indicated.

It is within the invention to transmit the narrowed speech band otherwise than directly over a line; for example, over a radio or carrier channel. It is also within the invention to modulate the band 500 to 2860 cycles down to a lower position in the frequency spectrum, such as 100 to 2400 cycles and to transmit it at such lowered frequencies. This may be advantageous where the attenuation is excessively high at the upper edge of the band. The band is then raised in frequency at the receiver.

What is claimed is:

1. The method of increasing naturalness in received speech comprising deriving low frequency components from the higher frequency components of speech waves and blending the derived low frequency components with the speech waves. I

2. The method of speech transmission comprising suppressing the lower frequencies of speech from transmission and restoring the effect of these suppressed components at a receiving point by deriving similar components from the frequencies actually transmitted.

3. The method of speech transmission comprising transmitting a band representing speech frequencies extending from 400 or 500 cycles upwards, deriving from this range components having frequencies in the region of 250 cycles and adding the derived components at a receiving point to the waves that are transmitted.

4. In a telephone system, a line, a speech transmitting circuit and a speech receiving circuit connected thereto, means in the speech transmitting circuit to eliminate from the speech Waves before. they reach the line,'the lower frequency components, and means in the receiving circuit for deriving from and adding to the received speech waves, components having the frequencies of those eliminated in the transmitting circuit.

JOHN C. STEINBERG. 

